Prosthetic acetabular cups are well known in the industry for replacing a portion of a patient's hip joint during total hip arthroplasty. One form of acetabular component includes a metal cup and a polyethylene liner secured therein by a variety of mechanical fasteners. The metal cup is affixed to the prepared acetabulum either by mechanical fasteners or by cement. Some metal cups include a porous surface layer to provide a structure for bone growth to enhance fixation of the cup. The polyethylene liner forms the articular surface for mating with the head of the femur or implant and are connected to the metal cup by a variety of fastening devices.
Once such device for retaining the liner within the cup is a round lock ring which is carried within a groove formed in the metal cup and communicates in an interference fit with a groove formed on the exterior of the polyethylene liner. This type of cup assembly is generally packaged with the lock ring and metal cup connected and the liner packaged separately. During the surgery, the liner is placed into the metal cup by pressing the liner into the cup and thereby spreading the lock ring slightly. The lock ring snaps into the groove formed on the exterior of the liner when properly aligned. The lock ring has a generally constant diameter or, in other words, is circular.
A problem exists with this type of construction since the lock ring is free floating within the groove of the metal cup prior to insertion of the liner. As the ring floats to one side, the other side of the ring is no longer supported by the metal shell. This can result in disassembly of the ring or difficult assembly of the poly liner.